Solid electrolytes based on zirconia (ZrO2), thoria (ThO2), and ceria (CeO2) doped with lower valent ions have been extensively studied. The introduction of lower valent ions, such as rare earths (yttrium (Y), lanthanum (La), neodymium (Nd), dysprosium (Dy), etc.) and alkaline earths (strontium (Sr), calcium (Ca), and magnesium (Mg)), results in the formation of oxygen vacancies in order to preserve electrical neutrality. The presence of the oxygen vacancies in turn gives rise to oxygen ionic conductivity (OIC) at high temperatures (e.g., greater than 800° C.). Typical commercial or potential applications for these solid electrolytes includes their use in solid oxide fuel cells (SOFC) for energy conversion, oxygen storage (OS) materials in three-way-conversion (TWC) catalysts, electrochemical oxygen sensors, oxygen ion pumps, structural ceramics of high toughness, heating elements, electrochemical reactors, steam electrolysis cells, electrochromic materials, magnetohydrodynamic (MHD) generators, hydrogen sensors, catalysts for methanol decomposition and potential hosts for immobilizing nuclear waste.
Both CeO2 and ThO2 solid electrolytes exist in the cubic crystal structure in both doped and undoped forms. In the case of doped ZrO2 partially stabilized ZrO2 consists of tetragonal and cubic phases while the fully stabilized form exists in the cubic fluorite structure. The amount of dopant required to fully stabilize the cubic structure for ZrO2 varies with dopant type. For Ca it is in the range of 12-13 mole %, for Y2O3 and Sc2O3 it is greater than 18 mole % of the Y or scandium (Sc), and for other rare earths (e.g., Yb2O3, Dy2O3, Gd2O3, Nd2O3, and Sm2O3) it is in the range of 16-24 mole % of ytterbium (Yb), Dy, gadolinium (Gd), Nd, and samarium (Sm).
Solid solutions consisting of ZrO2, CeO2 and trivalent dopants are used in three-way-conversion (TWC) catalysts as oxygen storage (OS) materials and are found to be more effective than pure CeO2 both for higher oxygen storage capacity and in having faster response characteristics to air-to-fuel (A/F) transients. In the automotive industry there is also great interest in developing lower temperature and faster response oxygen sensors to control the A/F ratio in the automotive exhaust. Additionally, reports concerning the use of ceria-based catalysts for soot oxidation and selective catalytic reduction of NOx with ammonia reveal new uses for solid solutions of CeO2 with other elements where low temperature Ce4+<- - -> Ce3+ redox activity probably does not have the critical importance that this redox activity has in TWC catalysis.
Oxygen storage (OS) in automotive catalyst applications arises due to the facile nature of Ce4+<- - -> Ce3+ redox cycle in typical exhaust gas mixtures. Benefits of yttrium and other rare earth doped CeO2—ZrO2 solid solutions compared to undoped CeO2 and CeO2—ZrO2 for TWC catalyst applications is due to improved Ce4+ reducibility at relatively low temperatures and enhanced oxygen ion conductivity (OIC), i.e., mobility of oxygen in the oxygen sublattice. These characteristics of the above mentioned solid solutions make them efficient in providing extra oxygen for the oxidation of hydrocarbons (HC) and carbon monoxide (CO) under fuel rich conditions when not enough oxygen is available in the exhaust gas for complete conversion to carbon dioxide (CO2) and water (H2O). Solid solutions with cubic structures were found to have advantages over other crystal structures.
The OS/OIC function is significantly enhanced by platinum group metals (PGM) such as palladium (Pd), platinum (Pt), and rhodium (Rh). In the presence of the precious metals, the reduction of the Ce4+ to Ce3+ in doped CeO2—ZrO2 solid solutions occurs at lower temperatures and improves TWC catalyst efficiency in reducing HC, CO, and nitrogen oxides (NOx) pollutants.
New emission regulations impose stringent requirements on catalyst durability, for example, 120,000 miles durability in the US and 160,000 kilometers (km) durability in Europe. One of the disadvantages of the current state of the art of yttrium and rare earth doped CeO2—ZrO2 solid solutions is loss of the OS/OIC properties as a function of aging due to segregation of the OS/OIC function from the PGM in typical TWC compositions. Thus improvement of the OS function of solid solutions is needed to maintain catalyst performance in the aged state. Therefore, one of our strategies was to enhance and stabilize the OS function by adding dopants that increase oxygen availability from OS/OIC at low temperatures, even in the absence of the PGM.
The new regulations also require control of soot and NOx emissions from diesel engines whose exhaust gas is essentially always net oxidizing. Mixed oxides of ceria may be useful for these conditions even though the reducibility of ceria that can occur under net reducing conditions has little relevance given the oxidizing nature of diesel exhaust gas. The origins of benefits of ceria in these emerging applications is not as well understood as in the case of established TWC catalyst applications.
What is needed in the art are OS/OIC materials having stable crystal structures, more specifically, cubic, as well as facile and high oxygen storage and oxygen ion conductivity properties.